The present invention is directed to a method and apparatus for the low cost production of an electret material.
Materials known as "electret" are those prepared by maintaining a film, sheet, or other shaped particle of a plastic composed of a polymeric or organic material such as carnauba wax or naphthalene; or polar resin such as polyfluorovinylidene rosin, a polyvinyl chloride rosin, polycarbonate, polyester, and acrylic rosin, etc.; a non-polar rosin such as polyethylene, polypropylene, polystyrene, etc. or copolymers of them; or a mixture of these materials at a suitable temperature for a long period of time under the application of a D.C. potential and cooling the article to room temperature while maintaining the D.C. potential, resulting in the polarization of the material. Because such an electret can maintain its polarized state for a long period of time as well as have excellent workability, toughness and flexibility, the electret is widely used in transformers such as speakers and microphones and also in other specific fields. Prior art methods for producing electrets are disclosed in U.S. Pat. Nos. 3,885,301, 3,748,727 and 4,348,711. The methods for the production of electrets disclosed in the above cited references have required the need of a high voltage source for generating an electrical field which is applied across the shortest dimension of the electret to produce the polarized state of the electret, which requirement increases the cost of the electret. It is therefore a principal object of this invention to manufacture electrets at a relatively low cost. It is another object of this invention to produce electrets at a relatively high rate of speed.